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The effect of hyperfine interactions on ultracold molecular collisions: NH(^3Sigma^-) with Mg(^1S) in magnetic fields

We investigate the effect of hyperfine interactions on ultracold molecular collisions in magnetic fields, using ^{24}Mg(^1S) + ^{14}NH(^3Sigma^-) as a prototype system. We explore the energy and magnetic field dependence of the cross sections, comparing the results with previous calculations that neglected hyperfine interactions (Phys. Rev. Lett. 103, 183201 (2009)). The main effect of hyperfine interactions for spin relaxation cross sections is that the kinetic energy release of the dominant outgoing channels does not reduce to zero at low fields. This results in reduced centrifugal suppression on the cross sections and increased inelastic cross sections at low energy and low field. We also analyze state-to-state cross sections, for various initial states, and show that hyperfine interactions introduce additional mechanisms for spin relaxation. In particular, there are hyperfine-mediated collisions to outgoing channels that are not centrifugally suppressed. However, for Mg+NH these unsuppressed channels make only small contributions to the total cross sections. We consider the implications of our results for sympathetic cooling of NH by Mg and conclude that the ratio of elastic to inelastic cross sections remains high enough for sympathetic cooling to proceed.